The GM2 gangliosidoses are severe neurodegenerative disorders characterized by impaired GM2 ganglioside degradation as a consequence of beta-hexosaminidase A deficiency. Mutations in the HEXA and HEXB genes, which encode the subunits of beta-hexosaminidase A, cause Tay- Sachs and Sandhoff disease, respectively. A third disease is caused by mutations in the GM2A gene encoding the GM2 activator protein. The GM2 activator protein forms a substrate-complex with the ganglioside enabling degradation by beta-hexosaminidase A. In humans, both the neurologic phenotype and neuropathology in the three diseases are very similar. Through targeted gene disruption in embryonic stem cells, we have previously created mouse models corresponding to Tay-Sachs and Sandhoff diseases. Unlike the human diseases, the mouse models showed dramatically different phenotypes as a result of differences in the ganglioside degradation pathway between mice and humans. We have now established mice with a disrupted Gm2alpha gene as a model for the third form of the GM2 gangliosidoses. The Gm2alpha gene disruption resulted in a null allele as evidenced by the absence of a Gm2alpha transcript in the knockout mice. Although the brain ganglioside level was elevated, ganglioside feeding experiments in fibroblasts indicated that the degradation pathway was not completely blocked in the Gm2alpha -/- mice. As of 18 weeks of age, there were no significant differences in motor function when mutant mice were compared to wild- type or heterozygous mice. At this age, the Sandhoff disease but not the Tay-Sachs disease mice were severely affected. The three gangliosidosis mice provide models for the human disorders allowing for a more detailed understanding of their pathobiology and for evaluation of potential therapies. We have also produced mice with both beta-hexosaminidase genes disrupted that are totally deficient in all three forms of the lysosomal enzyme (A, B and S). These totally beta-hexosaminidase deficient mice displayed facial and physical dysmorphia and severe movement difficulties, and had a short life span (1-4 months). In addition to gangliosidosis, the double knockout mice showed pathologic and biochemical features of mucopolysaccharidosis including dysostosis mutiplex. The novel phenotype of the totally beta-hexosaminidase deficient mice shows that GAGs like gangliosides are critical substrates for beta-hexosaminidase and that their lack of storage in Tay-Sachs and Sandhoff diseases is due to functional redundancy in the beta-hexosaminidase system.